Well, people on the internet can say whatever they want, it's having some kind of evidence that backs it up. The reading I've done very clearly puts the use of sandbox as coming from the video game world (they are way further advanced in analysis of techniques) and being adopted into use by the TTRPG community. If you claim you used it, ok, but it's pretty clear that your observance was of a small corner of the hobby and that such usage wasn't at all widespread. If it were, we'd see it's use in the contemporary writings, of which there are many and that focus largely around topics where "sandbox" would be a useful term. It's not there, so it wasn't in wide use in the hobby. You essentially have the hipster claim of "I was doing that before it was cool." Bully, but not distinguishable from an internet tall tale.
Let's not forget that this whole side trek was initiated by you telling
@estar they were wrong. That doesn't align to your claimed goals here.